noun
Literal meaning: a massive, immovable rock. Often used in metaphors for stability.
その城は盤石の上に建てられた。
The castle was built on a huge rock.
彼の地位は盤石だ。
His position is rock-solid.
盤石な基盤の上に会社を築く。
Build a company on a firm foundation.
From Chinese 盤石 (pánshí), literally 'tray-like rock' or 'massive rock'. The kanji 盤 and 磐 both mean 'large rock' and are used interchangeably in Japanese.